This application relates to an alarm system such as that manufactured by the present assignees under the trademarks Interceptor and Vanguard, details of which are available from a number of prior issued patents by the Assignees including U.S. Pat. No. 7,333,681 (Murphy) issued Feb. 19, 2008 which describes a system for securing multimode fibers, U.S. Pat. No. 7,142,737 (Murphy) issued Nov. 28, 2006 which describes a system for securing single mode fibers, and U.S. Pat. No. 8,233,755 (Murphy) issued Jul. 31, 2012 which describes a method for securing an optical communications network path by learning and suppressing ambient disturbances. The disclosures of each of the above patents are incorporated herein by reference.
In each of these systems, an optical signal is transmitted along a fiber and the signal received at a receiver which extracts a received signal which can have different characteristics relative to the transmitted signal due to movement of the fiber. The received signal is thus analyzed in a light signal analysis system to extract a signal indicative of any changes in characteristics of the light signal. This signal is then itself analyzed to determine whether the characteristics have changed sufficiently to indicate that a movement of the fiber indicative of an attempt to intrude into the fiber has occurred. The system further includes a control and alarm system which controls the system and an alarm in the event that an intrusion has been found.
The arrangement and location of the components can vary widely with the receiver located at the same end as the transmitter or at an opposed end. The alarm and control system can also be located at different positions in the system. Communication of data between the components can be carried out in different ways.
Many different constructions and techniques for signal analysis for this system are well known to persons skilled in the art and can be determined from one or more of the patents of the Assignees.
According to US Government regulations, a network carrying unencrypted classified data must be protected by a Protected Distribution System (PDS), One form of PDS is the Alarmed Carrier, which is a system by which a conveyance or carrier is alarmed by a device for detecting intrusions and attempted intrusions.
An example of an arrangement of this type is shown in U.S. Pat. No. 7,706,641 issued Apr. 27, 2010 by the present Applicants which describes in detail the monitoring system used in the present application, the disclosure of which is incorporated herein by reference. This patent describes that some or all of the optical fibers of a single-mode or multi-mode cable are monitored for intrusion by transmitting through the fibers a signal which can be analyzed for changes in its characteristics which are indicative of movement as a prelude to an intrusion event.
Data can be stolen from an optical fiber by removing the jacket and installing a tap device on the fiber or by other methods. Optical fiber intrusion detection systems of the type described above detect when a fiber cable is being subjected to vibration, motion, or handling that would be typical of an intrusion attempt. The system reports the intrusion attempt to the cable owner so that the cable can be inspected and the threat removed.
The fiber intrusion detection system works by transmitting a monitoring signal through a fiber as a single fiber or as a loop. Disturbances on the fiber cause the monitoring signal to be modulated. The modulated signal is received, digitized and processed, and alarms are raised when an intrusion is detected.
In the arena of sensor and transducer monitoring, a filtering mechanism may be needed to minimize false positive alarms that may be caused by spurious disturbances. It is important to recognize the occasional alarm and record it for later evaluation as needed without raising a full security alarm condition.
One example of a system of this type is disclosed in U.S. Pat. No. 9,455,999 issued Sep. 27, 2016 by Sohn et al, the disclosure of which is incorporated herein by reference. In this arrangement, there is provided a method in which events that do not themselves exceed a threshold are counted within a time limited buffer. That is the system only counts events that do not individually meet a first amplitude threshold indicative of a fiber break or disconnect so that the system requires a two stage test for the events. First, does the event itself meet a first amplitude threshold indicative of a fiber break or disconnect and, only if it does not meet that threshold, the system acts to count the number of such events and to trigger an alarm if the number of events counted within a predetermined time period exceeds a second threshold.